1. Field of the Invention
The present invention relates to a hard magnetic film structural body, a magnetoresistance effect device thereof, a magnetic head thereof, a magnetic recording/reproducing head thereof, a magnetic record medium thereof, and magnetic storing apparatus thereof.
2. Description of the Related Art
In magnetic recording apparatuses such as HDDs, record track widths thereof have been decreased so as to increase record densities. To compensate the decrease of the reproduced output due to the decrease of the record track widths, magnetic heads having high sensitive magnetoresistance effect devices (MR devices) have been used. These heads are referred to as MR heads. In particular, MR heads having spin valve films are hopeful successors in the next generation. The spin valve film is composed of a multi-layer magnetic film of which a first ferromagnetic film, a non-magnetic film, a second ferromagnetic film, and an anti-ferromagnetic film layered in the order. The magnetization of the first ferromagnetic film rotates corresponding to a signal magnetic. field (hereinafter, the first ferromagnetic-film is referred to as a magnetic sensible layer). The magnetization of the second ferromagnetic film is fixed by a bias magnetic field of the anti-ferromagnetic film. Hereinafter, the second ferromagnetic film is referred to a fixed magnetization layer. The spin valve film has a giant magnetoresistance effect (GMR).
In an MR head having a spin valve film Barkhausen noise due to a magnetic domain wall on the magnetic sensible layer is becoming a practically serious problem to be solved. In other words, when an external magnetic field is applied to a magnetic sensible layer having a variety of magnetic domains rather than a controlled magnetic domain, the magnetic directions of the individual magnetic domains are arranged to one direction at a time. At this point, a noise (Barkhausen noise) takes place in the output waveform. To remove the Barkhausen noise, a so-called abutted junction type MR head of which a hard magnetic film 2 such as a CoPt film is disposed on both sides of a spin valve film 1 as shown in FIG. 36 has been proposed. The magnetic domains of the magnetic sensible layer are controlled by a bias magnetic field (vertical bias) of the hard magnetic films 2 disposed on both sides of the magnetic sensible layer. When a variety of magnetic domains of the magnetic sensible layer are controlled to a single magnetic domain with such a bias magnetic field, the Barkhausen noise can be suppressed.
In the MR head shown in FIG. 36, a pair of electrodes 3 that supply a sense current are formed on the spin valve film 1. The spin valve film 1 is sandwiched by a pair of upper and lower magnetic shield layers 6 and 7 disposed through magnetic gap films 4 and 5, respectively. Thus, a shield type MR head is structured.
In this case, the hard magnetic film 2 preferably has a stably large coercive force Hc so as to stably maintain the magnetic domain control of the magnetic sensible layer for a long time. In addition, the hard magnetic film 2 preferably has a large residual magnetization Mr so as to properly apply a bias magnetic field to various magnetic sensible layers. However, in conventional MR heads, the film thickness of the hard magnetic film cannot be sufficiently increased due to a structural reason. In addition, the crystal structure of the hard magnetic film cannot be sufficiently controlled. Thus, the coercive force Hc and the residual magnetization Mr of the hard magnetic film are insufficient.
Moreover, as the densities of the magnetic recording apparatuses increase, the floating distance of the MR head from the magnetic record medium tends to decrease. In reality, it is predicted that the MR head is used in a low floating state, a pseudo-contacting state, and finally a contacting state. When the distance between the MR head and the record medium decreases, a magnetization reversal tends to take place in a hard magnetic field with a low coercive force. When the magnetization reversal takes place in the hard magnetic film, the magnetization of the magnetic sensible layer becomes unstable, thereby forming magnetic domains. As described above, the magnetic domains formed on the magnetic sensible layers result in the Barkhausen noise.
As countermeasures for increasing the coercive force of a hard magnetic film, the crystal characteristics thereof is improved with a very thick non-magnetic base film with a film thickness of for example 100 nm or more for increasing the effect of the base film (seed layer). The improvement of the crystal characteristics of the hard magnetic film contributes to the increase of the magnetic anisotropy. Thus, the coercive force of the hard magnetic film can be increased. Such countermeasures can be applied to a hard magnetic film as a magnetic record layer of a magnetic record medium.
However, in the abutted junction type MR head, when the magnetic gap narrows for a high density, a bias magnetic field leaks to a magnetic shield layer, thereby decreasing the effective bias magnetic field applied to a magnetic sensible layer. In addition, when a thick non-magnetic base film is used, in the abutted junction type, since the thick non-magnetic base film is disposed between a hard magnetic film and a spin valve film, the bias effect against the magnetic sensible layer decreases.
As countermeasures for improving the coercive force of a hard magnetic film, the film thickness thereof may be increased so as to compensate the bias magnetic field. However, in the magnetic head having a narrow track for high density recording, the increase of film thickness of the hard magnetic film results in the decrease of sensitivity. When a hard magnetic film that is thicker than a spin valve is used, the center portion of the film thickness of the hard magnetic film becomes close to a fixed magnetization layer. Since an anisotropic magnetic field of an anti-ferromagnetic film that fixes the magnetization of the fixed magnetization layer is weak, the magnetization reversal of the fixed magnetization layer tends to take place in the vicinity of the interface between the hard magnetic film and the anti-ferromagnetic film. The magnetization reversal of the fixed magnetization layer results in a noise.
Instead of the above-described abutted junction type, for example as shown in FIG. 37, a bias type of which a spin valve film 1 spreads on a hard magnetic film 3 and thereby a magnetic sensible layer of the spin valve film 1 and the hard magnetic film 2 are exchange-coupling has been proposed. In such a head structure (overlaid structure), after a thick hard magnetic film 2 is etched by for example ion milling process, a spin valve film 1 is formed. Thus, the surface of the lower magnetic gap film 4 that has been etched becomes rough. When the spin valve film 1 is formed on the lower magnetic gap film 4 that is not flat, magnetic characteristics become unstable such that the anisotropic magnetic field Hk becomes unstable, the coercive force Hc takes place in the direction of difficult axis, and the inter-layer coupling magnetic field Hin between the magnetic sensible layer and the fixed magnetization layer increases. Such an instability of the magnetic characteristics results in the Barkhausen noise. The surface roughness of the lower magnetic gap film 4 also takes place when the surface roughness of the hard magnetic film 2 is transferred by the ion milling process.
In addition, as shown in FIG. 38, even if the coercive force Hc (M point) of the hard magnetic film (in the overlaid structure) is large, when the residual magnetization Mr is low, the total coercive force Hc (L point) of the GMR film (such as a spin valve film) and the hard magnetic film decreases, thereby increasing the occurrence of the Barkhausen noise. In the hard magnetic film of conventional the MR head, the sufficient residual magnetization Mr has not been obtained.
As described above, in the MR head having the spin valve film, it is strongly desired to improve magnetic characteristics such as the coercive force Hc, residual magnetization Mr, saturated magnetization Ms, and square ratio S of the hard magnetic film without need to use a thick non-magnetic base film. As with the MR head having the spin valve film, in an MR head having a magnetoresistance effect film (AMR film) with the anisotropic magnetoresistance effect (AMR), a hard magnetic film is used to apply a horizontal bias. In this AMR head, likewise, it is desired to improve magnetic characteristics such as the coercive force Hc, residual magnetization Mr, saturated magnetization Ms, and square-ration S of the hard magnetic film.
Furthermore, a magnetic storing apparatus such as a magnetoresistance effect random access memory (MRAM) having a spin valve film has been studied. In this case, a satisfactory bias magnetic field is required.
On the other hand, in the hard magnetic film as the magnetic record layer of the magnetic record medium, to accomplish a low noise in high density magnetic recording, the value of [Mr·t (residual magnetization Mr×film thickness t)] should be small. However, in the Co type hard magnetic film, when the film thickness thereof is 10 nm or less, good magnetic characteristics cannot be obtained. In reality, the coercive force Hc, the square ratio S, and so forth are degraded. For example, when the film thickness t of the Co type hard magnetic film is decreased so as to decrease the value of (Mr·t), the crystal characteristics are degraded. In this case, the anisotropic magnetic field Hkgrain of the crystal grains degrades, thereby decreasing the coercive force Hc.
To accomplish the above-described low floating state, pseudo-contacting state, and contacting state of the magnetic head, the surface flatness of the magnetic record medium should be improved. To improve the surface flatness of the hard magnetic film as the magnetic record layer, the film thickness of the hard magnetic film including the film thickness of the base film should be decreased. Generally, when the film thickness of the base film is large, the diameters of crystal grains often increase. When the diameters of crystal grains become large as the film thickness increases, the boundary of crystal grains of the film surface becomes rough. Further, even if the boundary of crystal grains does not increase as the film thickness increases, crystal grains become column-like and the surface roughness of the film is large when the film thickness is large. (In any case,) when the film thickness of the base film is large, the diameters of crystal grains become large.
However, as described above, to improve the crystal characteristics of the Co type hard magnetic film, a base film as thick as 100 nm is required. This is because when the thickness of the base film is small, the crystal structure of the Co type hard magnetic film having a hcp structure cannot be satisfactorily controlled. In reality, the axis c of the Co type hard magnetic material cannot be oriented to the surface thereof.
The film thickness of the base film of the Co type hard magnetic film should be decreased so as to accomplish high density recording. As described above, when the film thickness of the base film is large, the diameters of crystal grains of the Co-type hard magnetic film become large. Thus, the number of magnetic grains per unit bit decreases, thereby causing the noise to increase. However, when the film thickness of the base film is decreased, the effect thereof is degraded. Thus, the base film cannot provide the Co type hard magnetic film with good hard magnetic characteristics.
Consequently, in the hard magnetic film as a magnetic record layer of a magnetic record medium, even if the film thickness of the hard magnetic film is small, without need to use a thick base film, it is strongly required to improve characteristics such as the coercive force Hc, residual magnetization Mr, saturated magnetization Ms, square ratio S, and so forth.
On the other hand, in the field of the magnetic record mediums, a surface magnetic record medium having a hard magnetic film with a bi-crystal structure has been expected as a low noise medium. The bi-crystal structure represents that sub-grains are present in each main-grain. In a main-grain, the surface components of the axis c of the sub-grains are perpendicular to each other. Since sub-grains function as magnetic grains, a low noise can be accomplished. In addition, a large coercive force can be obtained.
However, in the bi-crystal structure, a good fabrication condition thereof has not been established. It is said that the base film should have a bcc (body-centered cubic) (200) orientation. In the bcc structure, normally a plane (110) is the densest plane. Thus, on a base film that is simply formed, the bi-crystal structure cannot be obtained. Conventionally, the substrate should be heated at a temperature of at least 200° C. when the film is formed. However, even in such a process, the bi-crystal structure cannot be obtained with high reproducibility.